The proper procedure for a batch fermentation is first to inoculate a small flask of nutrient broth with a pure culture from a Petri dish, a culture tube (containing liquid nutrient), or a slant tube (containing solid gel). The inoculated flask is constantly agitated in a temperature controlled flask shaker. A small amount of the culture in the original flask is pipetted out during the exponential growth phase, or log phase, and is used to inoculate the next flask. This process is repeated a few times to ensure that the culture is acclimated before it is employed to study the fermentation kinetics. A similar process of repeated inoculation is carried out in the fermentation industry to build up enough inoculum needed to seed a larger fermentor. To reduce the shock resulting from a drastic change in the growth environment, the composition of the media used in preparing the inoculum should optimally be identical as that used in the main process.
When working with a pure culture, one must operate under the assumption that contaminating microorganisms are present everywhere in the open environment, a fact demonstrated in our previous experiment. It is important to know intuitively when sterile tools or glassware must be used and when sterilization is not necessary. This requires the ability to distinguish clearly the sterile side from the nonsterile side. In this experiment, the interior of the shaker flask is the sterile portion of the system. Anything that is that part of the system and anything that ever comes in direct contact with that part of the system must be sterile. Thus, the nutrient in the shaker flask before inoculation must be sterile, which in turn requires that the reservoir storing the filtered nutrient is sterile and that the entire process of dispensing the nutrient from the medium jar to the shaker flask is carried out aseptically. In addition, items that enter the shaker flask such as the cotton plug, inoculation loop, sampling pipets, and even air must all be sterile.
Most practical industrial fermentation processes are based on complex media because of the cost and the choice of the nutrients and the ease of nutrient preparation. For example, complex media for yeast fermentation can be easily prepared in a lab by following the same recipe as that used in the YPG agar, minus the agar: 5g/l yeast extract, 10g/l Peptone, and 5g/l glucose. However, the use of complex media is discouraged in the fundamental studies of fermentation kinetics because of the possibility of variations in the nutrient composition from run to run. For example, the exact content of a yeast extract preparation is not known, and its nutritional quality may vary from batch to batch. On the other hand, a defined medium can be reproduced time after time to ensure the reproducibility of biochemical experiments. The disadvantage of a defined medium is that there is always the possibility of missing some important growth factors. The formulation of a defined medium is often a tedious process of trial and error. However, a well formulated defined medium can support the healthy growth and maintenance of cells as effectively as, or sometimes superior to, a complex one. A defined medium will be used in this experiment.